ABSTRACT: Temporal changes in abundance of the heterotrophic flagellate Collodictyon triciliatum and its grazing rates on Microcystis aeruginosa were examined in a hypertrophic pond during a bloom of the cyanobacterium in 2000. The
rates were estimated from ingested cells of M. aeruginosa in food vacuoles of C. triciliatum and experimentally determined digestion rates of the flagellate feeding on the cyanobacterium. Cell densities of C.
triciliatum fluctuated between 10 and 510 cells ml-1, and the changing pattern was different from that of M. aeruginosa. Single cells and small colonies of M. aeruginosa were found in food vacuoles of the
flagellate, ranging between 2.9 and 16.7 cells flagellate-1. The digestion rate of C. triciliatum was determined as 0.74 ± 0.02% cell contents min-1. Estimates of grazing rate on M. aeruginosa by
C. triciliatum ranged between 1.1 and 9.9 M. aeruginosa cells flagellate-1 h-1, and grazing impact on the cyanobacterial standing stock fluctuated between 0.002 and 0.09% h-1 in the pond. It is
estimated that consumption of M. aeruginosa supplies carbon to the flagellate at a rate of 5.5 ± 3.2% to its cellular carbon content per hour. The growth rates of the flagellate are estimated as 0.016 ± 0.009 h-1, which overlap
those in previous laboratory studies. Hence, grazing on M. aeruginosa by C. triciliatum is probably of minor importance for the loss of cyanobacterial abundance in the pond, though the cyanobacterium is possibly an important
food for the flagellate.